The phrase "to stop the shooter" means to take action in order to prevent or halt a person who is actively involved in a shooting or intending to harm others with a firearm. Full definition
In that case, armed security attempted to stop the shooter from entering but were unsuccessful. (cochranfirm.com)
On Tuesday, for one, a school resource officer reportedly stopped a shooter at Great Mills High School in Maryland. (vox.com)
They advance the idea that guns should be carried by teachers, administrators, and security guards, regardless of those people's backgrounds or beliefs, because they think you can stop a shooter with a quick draw and some target shooting practice. (mommyish.com)